Transport of materials can also be gradual or sudden, and can exhibit a wide range of transport modes such as rock or debris falling; rock or debris tumbling; material clump tumbling or sliding; fluid flow; or some combination of any of these modes. Note: This Tabletop Exercise specifically relates to the landslide hazards associated with earthquakes; however, landslides describe many different types of motion of sand, soil, mud, rocks, and other earth structures.
Landslides occur in response to a wide variety of natural and manmade triggering events, such as earthquakes, heavy rainfall, volcanic activity, or road and building construction.
There are more variations in landslide mechanisms and triggering events than can be covered in an introductory course. Therefore, the most important outcome of any lesson on landslides is that students develop the curiosity and willingness to critically evaluate what their surroundings can tell them about landslides.
Read the following scenario while stopping to ask questions and discuss the material with your students at the indicated points or when students ask questions that are relevant to the discussion of earthquake hazards:. Three friends Sara, Amira, and Gozen live in the small city of Shahrabad, which is located in a beautiful mountain valley. The bottom of the valley has a small river running through it.
The walls of the valley have land that includes forests and farms. The friends have lived there since they were young and they know that earthquakes sometimes happen there. They have only felt one small earthquake, but their parents and grandparents have told stories about some strong earthquakes that have happened in the area.
Sometimes, during extreme weather like heavy snow or rain, the road that comes into Shahrabad from a nearby city is closed because rocks have fallen on the road or the road has washed away.
Sara and Amira live next to each other on farms located on slopes in the valley. Sara's farm used to have a natural spring at a crack between two rocks that produced drinking water for both Sara's and Amira's families, but the spring stopped producing water about a year ago.
Recently, a neighbor has started complaining that some parts of his land have become very soggy and soaked with water, especially near the bottom of the valley.
Potential answers: Springs occur when water flows through cracks below the Earth's surface. The water can be a mix of rain water, water from underground channels that travel downhill toward the river, or water that is pushed up from deep underground in the deepest parts of the Earth, which has not ever before been to the surface. Sometimes springs located very close to each other on the surface of the Earth have completely different paths that the water in each follows.
The water that is soaking the neighbor's land may or may not be related to the water that used to come out of the spring; however, both of the changing events indicate that the land that Sara, Amira, and the neighbor live on is undergoing movements that may not be visible on the surface. The spring might have stopped because of some small change in the path of the water due to small movements of the ground, or because the source of the water has become empty.
The changes in the path of the water could have occurred deep in the Earth or just a couple of meters beneath where the spring is located. When the water flows through narrow cracks, very small shifts in the ground can stop the flow of water.
Sara's and Amira's farm share a wooden fence to keep their farm animals from wandering around. Sara and Amira often climb over the fence to play in the forest around their farm.
About three years ago, they noticed that the fence posts were sloped at an angle at one spot in the fence near their path to the forest, and they were concerned that climbing over the fence was pushing the fence over. They changed their path so they didn't have to climb over the fence and then gradually forgot about the sloping fence posts. But the fence posts continued to tip over, little by little, without anyone noticing the low part of the fence.
Until one day, about a month ago, a donkey got away by jumping over the low part of the fence. They helped their fathers fix the fence and straighten the fence posts so the donkey couldn't get away. Potential answers: There are many answers possible that don't relate to landslide hazards. The fence could be old and the wood falling apart. The donkey could be pushing on the fence to eat some tasty grass that grows outside of that part of the fence. But the ground could also be moving very slowly beneath the farm, causing the fence posts to point uphill over the years.
The fact that the spring stopped giving water may support this idea even further, especially if the path of the water to the surface was broken because the ground had shifted very slightly. Gozen lives down in the city in a house. Sometimes all of the friends gather there to have dinner and listen to the radio or watch television. From where her family eats dinner, they can see the river. Her father helps to build and fix pipes that move water for farmers in the valley, and he also helps to build and fix houses.
A wealthy man has just built a house above a very steep hill that has a beautiful view of the valley, and he even paid just to have electricity from the city strung on wires up the hill to the house.
But the rooms already have cracks in the walls on the side of the house near the steep hill. Some of the windows and doors have also become very difficult to open and close. Gozen's father has been working there the past few days and he jokes about how the wealthy man complains that his house was not built very well by workers from a nearby city.
Potential answers: Again, the wealthy man may be right and the walls were indeed poorly built. Oftentimes, houses also settle naturally as they age and cracks form as the house comes to rest on the ground. However, the cracks are forming on the walls on the sides nearest the steep hill, which may indicate that the part of the house that rests on ground above the steep hill may be on unstable ground that is slowly creeping down the hill.
Doors and windows can become difficult to open and close because the house is changing shape as the ground moves beneath it, causing the frames to become misshapen. Also, if the ground was naturally unstable prior to building the house, the added load of the new house may be speeding the rate of movement of the creeping slope. Unstable ground or ground that is creeping is much more likely to release during a triggering event such as an earthquake or heavy rainfall.
There are many ways to tell what the real cause of the cracks may be. Other indications, such as the bending of pipes, fences, footpaths, or roads, can be found to see if the ground is moving. If the ground is shifting, then electrical wires attached to polls in the ground near the edge of the hill will become very tight as the polls move with the ground. One day, the three friends decide to go play in the forest together. They travel farther up the hill than they had ever gone before.
They find a very interesting bunch of very tall trees whose trunks grow out of the ground at an angle before the trees turn straight and point up into the air like a normal tree figure 2. Some of the trees have such a sharp angle that the girls can sit in the angle of the trees like a comfortable chair with their feet dangling down the slope of the hill! Most of the trees are curved in the same direction in the middle. The three friends name it the Sideways Forest.
Figure 2: A common tree shape formed due to ground creep Potential answers: Trees always grow up toward sunlight, so presumably the trees initially grew at a different angle when they were young. The fact that the trees were all curved in the same direction, and that they were all located next to each other, might indicate that the ground beneath the Sideways Forest is all shifting in one direction.
The trees are all much older than the girls, implying that the ground has been moving for a very long time. Generally classified as mass movements of rock, debris, and soil down a slope of land. While landslides are a naturally occurring environmental hazard they have recently increased in frequency in certain areas due to human activity. Although there are many different causes of landslides, they all have two things in common.
Natural causes of Landslides:. There are two main types of weathering: chemical and physical. An example of chemical weathering is acid rain.
Caused mostly by the burning of fossil fuels, acid rain is a form of precipitation with high levels of sulfuric acid, which can cause erosion in the materials in which it comes in contact. An example of physical weathering is wind blowing across the desert playas. This process causes rocks to form a specific pyramid-like shape and they are called ventifacts.
Select from these resources to teach about the process of weathering in your classroom. During an avalanche, a mass of snow, rock, ice, soil, and other material slides swiftly down a mountainside.
An alluvial fan is a triangle-shaped deposit of gravel, sand, and smaller materials called alluvium. A pyroclastic flow is a dense, fast-moving flow of solidified lava pieces, volcanic ash, and hot gases. It is extremely dangerous to any living thing in its path. Join our community of educators and receive the latest information on National Geographic's resources for you and your students.
Skip to content. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. A landslide is the movement of rock , earth , or debris down a slope d section of land. Landslides are caused by rain , earthquake s, volcano es, or other factors that make the slope unstable.
Geologist s, scientists who study the physical formations of the Earth, sometimes describe landslides as one type of mass wasting. A mass wasting is any downward movement in which the Earth's surface is worn away.
Other types of mass wasting include rockfall s and the flow of shore deposits called alluvium. Near populated areas, landslides present major hazard s to people and property. Landslides have three major causes: geology , morphology , and human activity. Geology refers to characteristic s of the material itself. The earth or rock might be weak or fracture d, or different layers may have different strengths and stiffness.
Morphology refers to the structure of the land. For example, slopes that lose their vegetation to fire or drought are more vulnerable to landslides. Vegetation holds soil in place, and without the root system s of trees, bushes, and other plants, the land is more likely to slide away. A classic morphological cause of landslides is erosion , or weakening of earth due to water.
Debris flows often leave a trail of rubble in their wake, forming distinctive ridges or levees. Debris flows range in size from 1m to 10m across, and may carry up to several cubic metres of debris.
One-off debris flows may occur on open hillsides, but repeated debris flows are more common below gullies. Debris cones form where repeated debris flows build up. Many debris cones in the Highlands have periods of activity and quiet. To trigger them, suitable weather conditions are needed and enough mud and rock must have collected higher up in the gully floor. Creep is the slow downslope movement of material under gravity.
It generally occurs over large areas. Solifluction is a mixture of creep and flow, which forms distinctive sheets, terraces and lobes of debris and boulders. Solifluction sheets and lobes are found on steeper slopes where the process has moved loosened boulders and soil downslope. Some smaller solifluction features are still active most winters.
Such slumps and slides occur in very wet weather, when the near-surface soil and rock debris gets saturated with water, and slides and flows downslope. They can develop into more dangerous debris flows on steep ground, where the landslide may travel a long distance, churning up the debris into a slurry. Cairngorms Landscapes.
Managing Scotlands rocks and landforms to safeguard them for the future is an important part of conserving our natural heritage.
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